1. Field of the Invention
The present invention relates to an infrared sensor for measuring temperatures of objects by means of measuring radiation temperatures without making physical contacts with the objects.
2. Description of the Related Art
An infrared sensor basically comprises an infrared sensitive element for capturing infrared rays and a compensating element for obtaining a signal that is used as a basis for calculating the quantity of infrared rays detected. Such an infrared sensor has the structure that prevents infrared rays from entering the compensating element. In order to measure the quantity of infrared rays detected by the infrared sensitive element accurately based on the signal output from the compensating element, it is preferable that the compensating element and the infrared sensitive element have identical characteristics. Therefore, it is common to use the same material and shape for both the compensating element and the infrared sensitive element to equalize their characteristics.
However, the characteristics of these elements may differ from each other minutely due to variations in manufacturing conditions. When a sensor is miniaturized, in particular, and delicate temperature changes have to be measured accurately, it may become impossible to compensate the measured values due to the difference of their characteristics. Therefore, it is necessary to evaluate with each sensor if it is necessary to match the characteristics of the infrared sensitive element and the compensating element.
Japanese Patent Unexamined Publication JP-A-08-159866 (1996) discloses an infrared sensor comprising an infrared sensitive element for infrared rays, a compensating element for compensating the measured values of the infrared sensitive element, and a processing circuit all formed on a single substrate. The elements are maintained in a vacuum environment.
The characteristics of the infrared sensitive element and the compensating element are easily equalized as they are both formed simultaneously on the identical substrate. In other words, there is no need for further matching the characteristics between the infrared sensitive element and the compensating element to improve the accuracy of the measured values.
Moreover, the infrared sensor has the infrared barrier layer for covering the entire internal surface of a package body (comprising top and bottom covers) hermetically sealed to maintain a vacuum inside, and an infrared transmission aperture provided on an area of the infrared sensitive element side of the top cover. In other words, the design allows infrared rays to enter an element that functions as the infrared sensitive element but prevents infrared rays from entering another element that functions as the compensating element.
However, in a design where an infrared transmission area is provided in one area of the package body and the rays are prevented from entering the sensor through any other parts of the package body, it is necessary to minimize the area of infrared transmission in order to block infrared rays from the compensating element completely. This causes a problem because it limits the view angle of the infrared sensitive element and the quantity of incident infrared rays, thus reducing the sensitivity of the sensor itself. Also, the infrared rays entered through the infrared transmission area makes random reflections inside the package body and may inadvertently enter the compensating element though the quantity may be small. Hence, there still remained a problem of not being able to detect delicate changes of infrared rays in the prior art.